# Clouds of Fluffy Aggregates: How They Form in Exoplanetary Atmospheres and Influence Transmission Spectra

## TL;DR

This study models the formation and impact of fluffy, porous cloud particles in exoplanet atmospheres, revealing their role in elevating clouds and obscuring spectral features, with implications for interpreting exoplanet spectra.

## Contribution

It introduces a porosity evolution model for cloud particles, showing how fluffy aggregates form and influence transmission spectra, unlike previous compact-sphere assumptions.

## Key findings

- Fluffy aggregates can reach higher altitudes than compact particles.
- Cloud scattering causes spectral slopes and obscures molecular features.
- High-metallicity atmospheres with small monomers explain GJ1214 b's flat spectrum.

## Abstract

Transmission spectrum surveys have suggested the ubiquity of high-altitude clouds in exoplanetary atmospheres. Theoretical studies have investigated the formation processes of the high-altitude clouds; however, cloud particles have been commonly approximated as compact spheres, which is not always true for solid mineral particles that likely constitute exoplanetary clouds. Here, we investigate how the porosity of cloud particles evolve in exoplanetary atmospheres and influence the cloud vertical profiles. We first construct a porosity evolution model that takes into account the fractal aggregation and the compression of cloud particle aggregates. Using a cloud microphysical model coupled with the porosity model, we demonstrate that the particle internal density can significantly decrease during the cloud formation. As a result, fluffy-aggregate clouds ascend to altitude much higher than that for compact-sphere clouds assumed so far. We also examine how the fluffy-aggregate clouds affect transmission spectra. We find that the clouds largely obscure the molecular features and produce a spectral slope originated by the scattering properties of aggregates. Finally, we compare the synthetic spectra with the observations of GJ1214 b and find that its flat spectrum could be explained if the atmospheric metallicity is sufficiently high ($\ge100\times$ solar) and the monomer size is sufficiently small ($r_{\rm mon}<1~{\rm {\mu}m}$). The high-metallicity atmosphere may offer the clues to explore the gas accretion processes onto past GJ1214b.